My experience with granting a group of users access to nodes based on a custom field is this:
- I told them they needed to set the custom field. If they didn't do it, they would not see the node.
- They forget the need to set the custom field and end up calling me.
- I update the custom field and remind them of their responsibility.
- They get it right from that point forward.
The end result is after making some mistakes, people tend to correct their own mistakes. Give them a chance to fail and then they will succeed the next time.